The concentration of the detergent in the cleaning solution used with the precision cleaning devices of the above type often changes during cleaning due to the consumption or natural deterioration of the detergent, dilution with the water carried in by the work or added for the adjustment of the solution level and other causes. On such occasions, the concentration often falls below the lowest limit of its desirable range. Accordingly, suitable means for automatically monitoring and controlling the detergent concentration has been strongly desired.
To meet this requirement, an apparatus to control the percentage concentration of the detergent on the basis of the previously measured electrical conductivity (.mu.S) of the cleaning solution has been commercially available. This apparatus compares the electrical conductivity of the cleaning solution measured from time to time with a sensor with the previously determined electrical conductivity corresponding to the appropriate detergent concentration. The detergent feed pump is turned on and off according to the result of this comparison so that the measured concentration is always kept with the desired limits.
However, this conventional controller uses the as-measured electrical conductivity for concentration control. Therefore, the measured values are displayed and compared with the reference value preset in units of electrical conductivity. Consequently, it has been impossible to learn the detergent concentration directly from the measured electrical conductivity. In specifying the proper concentration range, in addition, it has been necessary to determine the electrical conductivity corresponding to the proper concentration as a controller input from the interrelationship therebetween. Thus, the conventional controller has not been easy to deal with. Besides, each detergent has its own inherent electrical conductivity that differs among different detergents. When a new detergent is used, therefore, its inherent interrelationship between concentration and electrical conductivity must be determined by experiment to that the electrical conductivity corresponding to its proper concentration can be derived therefrom.